1. Field of the Invention
This invention relates to a paper margin detecting device for use in a printing apparatus, and particularly to a paper margin detecting device for accurately detecting the position of a left margin of a print paper which is fed onto a platen using a sensor mounted to a carriage.
2. Description of Related Art
As one of conventional printing apparatuses, such as a wire dot printer, a thermal printer and a laser printer, there has been proposed a printing apparatus in which a compact type of paper detecting photosensor is provided to a carriage for moving a print head, and on the basis of a sensor signal from the photosensor it is detected whether a print paper has been fed onto a platen, that is, whether the print paper exists on the platen. In the printing apparatus as described above, even though the print paper is fed to a substantially predetermined position on the platen by aligning the left margin of the paper, there is enough variation in feed that the print paper which has been fed onto the platen is not precisely located at a fixed position, and thus the locating position of the print paper on the platen varies slightly for every print paper so fed.
Particularly in a case where a print paper fed onto the platen is required to have a very small margin amount (a small area of blank portion: for example, about 1 mm width) at the peripheral portion thereof in order to print characters or an image on substantially the entire surface of the print paper, it is essential that the left margin of the print paper be accurately detected. In order to satisfy this requirement, a paper margin detecting device containing a photosensor fixed to a carriage is provided so that the left margin of the print paper can be accurately detected by the paper margin detecting device.
In this device, the photosensor may be disposed at the same position as the print head in a printing direction, or may be disposed so as to be displaced from the print head by a predetermined distance in the printing direction or the opposite direction thereto. However, this device has a problem in that an installation or fixing error frequently occurs when the photosensor is installed or fixed to the carriage. That is, the photosensor is structured to emit a light beam from a light emitting element to be irradiated onto an object and the light beam reflected from the object is detected by a photosensitive element. However, with this structure various errors, such as an inclination error and a fixing-position error, of the sensor with respect to the platen, result directly in a detection error. In order to prevent occurrence of these errors, the sensor must be fixed to the carriage with high accuracy. Further, any adjustment of the fixing position of the sensor is very complicated, requiring a lot of time and labor. In addition, even if these errors are maximally suppressed, it is impossible to completely extinguish them and, thus, the accuracy of the paper margin detection is limited.